Interest in the percutaneous detecting of body analytes (i.e., fluid electrolytes), organics (e.g., glucose), pharmaceuticals and illicit drugs has grown over the years. In recent years, a number of electrochemical sensors have been developed for detecting each of these analytes in the blood or interstitial fluid of a patient. For example, glucose sensors have been developed for obtaining an indication of blood glucose levels in diabetic patients. Existing electrochemical sensors require either collection of a sample from the patient or some form of invasive insertion of a sensor probe into the patient.
Thin film electrochemical sensors have been developed for subcutaneous placement of sensor probes in direct contact with the patient's blood or other extracellular fluid. One such example of a thin film electrochemical sensor, disclosed in U.S. Pat. No. 5,391,250 issued to Cheney, II et al., is fabricated using thin film mask techniques. With thin film mask techniques, three thin film conductive elements are laid down in close parallel relation on a substrate and encased between flexible insulating layers of polyimide material. The conductive elements are left exposed at the distal end of the electrochemical sensor for placement in direct contact with the patient's blood. Appropriate electrode chemistries are applied to the exposed conductive elements for use as a blood glucose sensor. One of the exposed conductive elements has a coating containing glucose oxidase to define a working electrode. The other two exposed conductive elements are coated with other suitable materials or left uncoated to define a reference electrode and a counter electrode for the electrochemical sensor. The conductive elements are left exposed at the externally located proximal end for connection to a suitable monitor.
The exposed conductive elements at the distal end of the electrochemical sensor are transcutaneously placed using a sensor insertion set such as disclosed in U.S. Pat. No. 5,390,671 issued to Lord et al. The sensor insertion set comprises a separate slotted insertion needle extending through a mounting base that attaches onto the patient's skin. The thin film sensor has a proximal end carried by the mounting base and a distal segment with the exposed sensor electrodes thereon protruding from the mounting base. The proximal end of the sensor is linearly offset from the distal segment so that the distal segment can be fitted into the slotted insertion needle while the proximal end is carried by the mounting base. The distal segment is transcutaneously placed as the insertion needle pierces the patient's skin upon the mounting base being pressed onto the patient's skin. The insertion needle is then withdrawn over the electrode from the patient leaving the distal segment at the selected site and the mounting base on the patient's skin.
Insertion of the needle is comparatively invasive, painful and frightening to many patients. Therefore, there is a need for a minimally invasive, painless placement of electrochemical sensors in the patient's skin. Furthermore, it is desirable in some circumstances to apply the electrochemical sensors to individual skin-piercing elements rather than in close parallel relation on one sensor probe for improved manufacturability.